1. Field of the Invention
The present invention relates to anamorphic objective lenses, and more particularly to a range of different focal length anamorphic objective lenses providing traditional anamorphic imaging characteristics and having relatively simple optical surface shapes and corresponding potential minimized cost by having an arrangement of three lens groups with spherical, anamorphic and spherical powers and an optical stop located outside of the first and second lens groups.
2. Description of Prior Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Contemporary anamorphic objective lenses normally have an optical axis and are commonly based on a front anamorphic lens group having predominantly X cylinder refractive optical surfaces and a rear spherical lens group with an optical stop in the rear spherical lens group or between the rear spherical lens group and the front anamorphic lens group that may be in the form of a variable aperture diameter iris or diaphragm.
This anamorphic objective lens arrangement produces images having numerous residual optical aberrations and characteristics most of which are desired by cinematographers because they produce an artistic look that is different from spherical objective lenses.
Many of the less desired residual optical aberrations and characteristics of this arrangement were accepted by cinematographers with film based cameras but with the advent and adoption of electronic sensor based digital cameras some of them have become less acceptable. In particular the amount of residual chromatic aberration has become less tolerable whereas some field curvature combined with some residual astigmatism is still acceptable.
Specific anamorphic objective lens characteristics of this arrangement such as the oval or elliptically shaped of out of focus objects commonly referred to as the bokeh as compared to the spherical shape produce by spherical objective lenses is preferred because of the distinctive artistic look produced. In the case of an anamorphic objective lens that squeezes the horizontal field of view by substantially two times as compared to the vertical field of view, the ratio of the major axis to minor axis dimensions of an elliptically shaped bokeh is substantially two to one, which is commonly known as a traditional bokeh shape. Another characteristic that is desired because of the distinctive artistic look produced is the depth of field being different in the vertical azimuth direction of the field versus the horizontal azimuth direction of the field. In the case of an anamorphic objective lens that squeezes the horizontal field of view by substantially two times as compared to the vertical field of view, the depth of field in the horizontal azimuth direction of the field is substantially two times greater than the depth of field in the vertical azimuth direction of the field.
Improving the optical aberrations and characteristics of anamorphic objective lenses of this arrangement may involve increasing optical surface shape complexity and hence manufacturing cost including adding aspherical and free-form shaped optical surfaces.
Thus, to address the artistic need of cinematographers and maximize the imaging potential of both film and digital cameras a cost effective anamorphic objective lens arrangement with a suitable blend of residual optical aberration correction and characteristics needs to be achieved.